Because of evolving environmental regulations, notably Titles I and III of the 1990 Clean Air Act, the management of chemical emissions from practical natural gas flames may be required. Chemical emissions of concern include carbon monoxide CO; oxides of nitrogen, NO.sub.x, where x=1 or 2; acids of nitrogen HNO.sub.y, where y=2 or 3; formaldehyde, CH.sub.2 O; and air toxins, C.sub.m H.sub.n O.sub.z, where m=1-70, n=1-32, and z=0-12. The goal is to control these emissions simultaneously.
The present invention, on an unvented or vented gas-appliance burner, is intended to allow a variety of contemporary and future gas appliances to be operated with significantly reduced emissions to the indoor and outdoor atmosphere. By significantly reduced is meant that the concentrations of the designated pollutants are lowered to less than 10 parts per million (ppm).
Past attempts to accomplish the goal of simultaneous emissions control have not been successful. Continued lack of success may endanger the continued use of gas appliances for cooking, space heating, water heating, and other domestic, commercial, or industrial uses, as the increased use of electric appliances, which do not have the same heating element emissions problem, may be encouraged.
Some emissions are products of complete combustion, while others are products of incomplete combustion, or of other chemical reactions that take place in or near the flame, which poses a dilemma. Strategies for the control of one trace emission may be incompatible with strategies for the control of another. Often the concentration of one pollutant may be reduced significantly while that for another remains unchanged or even increases. Strategies for the control of single pollutants are somewhat understood and workable, but a basis for the simultaneous control of all unwanted combustion related emissions, particularly those identified here, has heretofore remained unclear .sup.(1-45).
Two general approaches are known for reducing emissions from gas-appliance burners .sup.(38). One involves adding an object to an appliance burner, usually without an accompanying change in operating conditions. The object, which glows red hot in the flame, is called a "radiant insert" and is either solid (flame gases flow around) .sup.(24) or porous (flames gases flow through) .sup.(26). In tests on gas appliances, both types of inserts have typically reduced NO by about 50%, NO.sub.2 by about 25%, and have caused either no change or an increase in CO. Emissions remained at double-digit ppm levels. The effects on the other pollutants of interest are not known. Hence, the ultimate objective of total emissions control has not been achieved by the use of radiant inserts.
The other approach involves replacing a conventional appliance burner with a new one, called a "radiant" burner. Instead of blue flames appearing at the ports of a relatively "cool" burner, no flames are apparent, and the burner glows red hot at about 1000.degree. C. In tests with radiant burners, NO emissions were reduced by as much as about 90%, to near single-digit ppm levels, but with increases, to double-digit levels, in NO.sub.2, CH.sub.2 O and HNO.sub.y (32,40,42,44,45).
Efforts were undertaken to confirm these findings .sup.(31,40). Confirmation of baseline emissions data, and their alteration upon the use of radiant inserts or burners, was necessary because of doubts about their accuracy. Emissions data are sometimes inadvertently biased or distorted by the methods by which they are determined .sup.(31). The results just reviewed were found to be correct; that is, they were not measurement-protocol specific. So they could be reliably used as baseline data.
A comprehensive review of the combustion literature indicated that although data about the ability of radiant inserts or burners to reduce emissions were valid, the rationales presented to explain their action were suspect. Ambiguities were found that led to the present invention. An analysis of fundamental combustion mechanisms also revealed a plausible and defensible explanation of why radiant inserts and burners might be effective at NO reduction, but little else. Moreover, this analysis also revealed how an innovative approach, counter to that taught by the prior art, might achieve the objective of total emissions control.
Flames are either of the diffusion or premixed type, depending on whether none, some, all, or more of the air required for complete combustion is mixed with the fuel before it reaches the burner outlet. This mixing is called primary aeration. If primary aeration is zero, a diffusion flame exits, burning where 100% of the air required for complete combustion becomes available. If the primary aeration is between 60 and 200%, the flammability limits for premixed natural gas, flames exist. When the primary aeration is about 60 to not quite 100%, premixed flames are called "partially premixed" or "fuel rich"; when 100%, they are called "stoichiometric" or "fully premixed"; and when greater than 100%, they are called "fuel lean" or "having excess air". Most gas-appliance burners operate with partially premixed flames .sup.(1,22,31,38).
Two distinctions are often overlooked regarding a partially premixed flame. First, it consists not of one flame, but two flames in series: an inner fuel-rich premixed flame, followed by an outer, stoichiometric diffusion flame. Second, allowable primary aeration can be less than 60%, because the downstream diffusion flame acts as a pilot. Most combustion research is conducted on single flames, even if partially premixed .sup.(1-12,14-21). The downstream diffusion flame is eliminated by burning the premixed flame in an inert atmosphere, usually nitrogen (N.sub.2).
These definitions and distinctions were critical to the conception of this invention, because their effect on strategies for emissions reduction appears not to have been properly taken into account, as will be explained after critical definitions and distinctions are made regarding emissions formation.
Flames are capable of oxidizing not only the natural-gas fuel to CO.sub.x, but also the N.sub.2, which constitutes about 79% of air, to NO. The NO formation mechanism has been the subject of considerable analysis .sup.(17). Research indicates that there are probably two mechanisms by which N.sub.2 is oxidized to NO in flames. Named for their discoverers, they are "Fenimore-NO" (F-NO) and "Zeldovich-NO" (Z-NO) .sup.(4,17). Several features of these mechanisms have been widely discussed and applied in simplified form without question or qualification. Noticing this lack of rigor, detail, and regard for proper application led us to the present invention and our understanding of its probable mechanism.
For example, it is widely accepted that Z-NO forms primarily downstream of the flame ("late"), has a positive temperature dependence (the hotter the flame, the more Z-NO), dominates at primary aeration at levels greater than 100%, and has the following chemistry .sup.(4,17,45) : EQU N.sub.2 +O=NO+N (1) EQU N+OH=NO+H (2) EQU O.sub.2 +N=NO+O (3)
It is also widely accepted that F-NO forms "promptly" in the flame, is independent of temperature, dominates at primary aeration levels less than 100% and has this chemistry (x=1-3): (17,23) EQU N.sub.2 +CH.sub.x =HCN+N (4)
followed by the oxidation of hydrogen cyanide (HCN) and Reaction (3) to convert the atomic nitrogen (N) to NO. Awareness of linkage between the formation chemistries of Z-NO and F-NO via the mutual N-radical also helped in the conception of the present invention.
This background information suggests a strategy for Z-NO, but not F-NO, control. That is, burn at a high level of primary aeration in a flame that is highly cooled. If the state of the art is assumed to be information presented at the 1992 International Gas Research Conference, this strategy has promise, as papers teach that near single-digit ppm NO levels are achieved by premixed radiant burners, operating at 130% primary air and burner surface temperatures of about 1000.degree. C..sup.(41,45).
While a strategy for just the Z-NO component of the total emissions-control problem might seem fairly straightforward, strategies for the others are not. The effect of Z-NO control on F-NO, NO.sub.2, HNO.sub.2, HNO.sub.3, CO, and CH.sub.2 O is somewhat known, and appears to be adverse. Data indicate that most, if not all, remaining NO may appear as NO.sub.2, and that HNO.sub.2 and CH.sub.2 O may become new emissions problems .sup.(40). The literature acknowledges that practicable strategies for F-NO control are not known .sup.(17,42,43,45).
An analysis was conducted to understand why NO.sub.2, HNO.sub.2, and HNO.sub.3 were traded for NO in the best available control strategy, radiant burning .sup.(45), and why radiant burner inserts could effect, at best, only a 50% reduction in NO and a 25% reduction in NO.sub.2 .sup.(24,26).
Whereas NO is formed early in a flame, via the oxidation of N.sub.2 in combustion air, NO.sub.2 is probably produced after the flame, via the oxidation of NO .sup.(13,19,21). Conversion of NO to NO.sub.2 is promoted by trace hydrocarbons (HCs), and by thermal quenching .sup.(21,25,27,29). The latter translates to lowered temperatures favoring NO.sub.2 formation, which supports the comment that while a strategy may reduce one pollutant, it may increase another: e.g., lowered temperatures reduce NO, but increase NO.sub.2.
One or the other of these NO.sub.2 -promoting conditions is inherent in conventional partially-premixed "high-NO" flames (FNO +Z-NO), and state-of-the-art fuel-lean "low-NO" (F-NO) ones. Any NO generated in the inner fuel-rich flame is exposed to HCs as it is transported into the outer diffusion flame. NO generated by an outer or single flame is subject to quenching via contact with secondary air. The result of these effects is to favor conversion of NO to NO.sub.2, which is the precursor to the N-acids, HNO.sub.2 and HNO.sub.3, and to maintain high in-flame concentrations of CO .sup.(26.
Initial research on the mechanisms for HNO.sub.2 and HNO.sub.3 formation suggests that these species are probably affected in the same manner as NO.sub.2. Lower, or lowered, temperatures favor their formation .sup.(16, implying that they are probably formed indirectly, via oxidization of F-NO derived NO.sub.2.
With this understanding in mind, it became apparent why the use of a radiant insert or burner would not be expected to cause a simultaneous reduction in NO, NO.sub.2, and N-acids. Any, if not most, of the NO not prevented from forming, would be readily converted to NO.sub.2 and the N-acids by the procedures used to suppress Z-NO formation. Hence, these species probably could only be reduced to ultra-low levels by eliminating nascent NO, or the F-NO.
The preceding analysis revealed a new strategy for total emissions control. An insert might be more effective if it is non-radiant, is inserted at an upstream flame position that is cool, and minimizes contact of NO and trace hydrocarbons with cool secondary air. This strategy suggested a highly porous member be positioned at the base, or cooler, region of a stoichiometric premixed flame. These conditions seem never to have been tested, as we could find no data for inserts in flames with primary aeration greater than 60%, or at positions early in any flame. However, speculation in one patent on a radiant solid insert states that if its position were in the near-flame zone, NO and CO emissions might increase, rather than decrease .sup.(24). The effect of inserting a porous structure into this near-flame zone was not known, but it seemed to us to be worth testing.
Experiments were conducted using an apparatus called a Uniburner, a device that allows the effects of burner design and operation on emissions to be reliably evaluated under welldefined and controlled and realistic conditions .sup.(31). Emissions were sampled using two different complementary techniques. A direct technique employed a sampling probe, and was conducted using a protocol the reliability of which had been validated via international interlaboratory testing .sup.(31). An indirect technique employed a chamber method, the results of which have also been validated .sup.(40). Data acquired using both methodologies, executed by independent researchers, were in agreement.
Baseline emissions were measured for a generic rangetop burner cap .sup.(31). The operating conditions under which this burner generated the highest concentration of total emissions was a firing rate of 9.4 KBtu/hour, which fixed its port loading at 36.2 KBtu/hour-inch.sup.2 and a primary aeration of 60% Under these baseline conditions, the partially premixed dual-blue flame emitted 70 ppm NO, 30 ppm NO.sub.2, and 100 ppm CO. Emissions for HNO.sub.2, HNO.sub.3 and CH.sub.2 O were already at single-digit ppm levels, 3, 1, and 0.5 ppm, respectively .sup.(40).
Within experimental uncertainty, as primary aeration was increased from 60% to 100%, with no change in firing rate or port loading, baseline emissions remain unchanged. Results elsewhere confirmed the same trend for NO and NO.sub.2 emissions from different rangetop burners, indicating that the baseline data obtained here were typical, and not rangetop burner-cap specific .sup.(38).
Different porous inserts were tested. Variables included thickness, with respect to flame height, and porosity, with respect to pressure drop. Favorable results were obtained using a porous member 0.25 inch thick with 94% porosity. Unfavorable results, that is, increases in emissions, especially CO, were obtained when porous inserts were made with lower porosity (81, 87, and 90%) or thickness (0.125) inch..sup.(46)
Emission tests produced encouraging and surprising results. At 100% primary aeration, where the non-radiant porous insert was expected to be effective, baseline emissions were reduced by 86% for NO, 89% for NO.sub.2, 81% for HNO.sub.2, 70% for CH.sub.2 O, and 74% for CO. These dramatic reductions, averaging about 80%, resulted in single-digit ppm emissions for all species except CO, or near-total emissions control.
A surprising result occurred at 85% primary air, the only intermediate level tested. At 85% primary aeration, at which porous inserts had not been expected to have any special effect, a composite reduction of about 50% was observed in NO, NO.sub.2, and CO emissions.
This last unexpected result implied that non-radiant porous inserts may have promise in appliances with "atmospheric" burners, whose venturis can achieve primary aeration as high as 85% .sup.(38). Although not total emissions control, as defined here, the partial overall reduction that can be achieved may extend the allowed lifetime of current gas appliances, if and when regulations requiring emissions control are enacted. Many current gas appliances do not require a fan to achieve their operating level of primary aeration. Effective emissions control for them could consist of the mere retrofitting of the gas appliance burner with a porous insert.
If the porous insert had effectively controlled emissions only at primary aeration of at least 100%, gas appliances would need both a porous insert and a fan, which would not be as technically straightforward or economically favorable. Because fans are being integrated more frequently into the designs of next-generation gas appliances, the use of the new porous-insert technology may remain a simple retrofit process in the future.
To summarize, the technical literature appears to contain no information on the concept of a non-radiant porous insert as an emissions control technology for blue flames on gas appliances.